1. Field of the Invention
This invention relates to an improved process for the preparation of organic isocyanates. More specifically, organic isocyanates are produced by a continuous hot phosgenation process which results in purer products in higher yields than previously achieved.
2. Description of the Prior Art
The majority of organic isocyanates is produced by phosgenation of primary amines, their hydrochlorides or their carbonamide acid salts, see, e.g., "Ullmanns Encyclopaedie der technischen Chemie," fourth edition, Vol. 13, p. 350 ff.
The so-called based phosgenation is technically the most important process. In a cold state, the amine present in an inert solvent reacts with excess phosgene in solution (cold phosgenation) according to the following reaction: EQU 2RNH.sub.2 +COCl.sub.2 .fwdarw.R--NH--CO--Cl+R--NH.sub.3.sup.+ Cl.sup.-
In the second step, the so-called hot phosgenation, carbamoyl chloride dehydrochlorinates at temperatures above 80.degree. C. to give hydrogen chloride and the desired isocyanate R--NH--CO--CL.fwdarw.RNCO+HCL and the hydrochloride is converted into the isocyanate EQU R--NH.sub.3.sup.+ Cl.sup.- +COCL.sub.2 .fwdarw.RNCO+3HCl
The two reactions can be summarized up as follows: EQU R--NH--CO--Cl+R--NH.sub.3.sup.+ Cl.sup.- +COCl.sub.2 .fwdarw.2RNCO+4HCl
A problem encountered in the above-described process is that there is a competitive reaction to isocyanate formation. The amine reactant and the already formed carbamoyl chloride or the isocyanate can react to form urea according to the following scheme. EQU R--NHCOCl+R--NH.sub.2 .fwdarw.R--NH--CO--NH--R+HCl EQU R--NCO+R--NH.sub.2 .fwdarw.R--NH--CO--NH--R
Depending on the specific amine, the ureas are converted into isocyanate only with great difficulty or not at all. This applies especially to aliphatic amines and particularly to polyamines. Therefore, the process must be conducted so that urea formation is prevented.
To avoid these problems it is necessary to conduct the prephosgenation reaction in preferably a large excess of phosgene and at temperatures as low as possible. Therefore, in practice a relatively concentrated phosgene solution in excess is used and the amine is added in an organic solvent.
It is critical that the amine be dispersed in the phosgene solution as quickly as possible. Therefore, special mixing devices are required. It is emphasized that it is important for the prevention of the secondary reaction of the amine with already formed isocyanates "that every single amine molecule introduced into the process would have to be immediately surrounded by phosgene before it gets into contact with any formed isocyanate molecules" (DE-PS No. 17 92 660, column 3, lines 15-19). This is practically impossible to achieve. The most effective mixing units are mixing nozzles. However, clogging and deposits of solids occur in the mixing nozzles which interfere with the mixing process.
Moreover, due to the fact that a concentrated phosgene solution is necessary, considerable effort is required to dissipate the reaction heat formed in the strongly exothermic reaction which occurs at a relatively low temperature level. Practically speaking, the operation is continuously adiabatic, i.e., at a relatively high temperature (cf. DE-PS No. 11 65 587). In addition, conducting the reaction in and maintaining a highly concentrated phosgene solution is questionable from a safety aspect. Despite the foregoing disadvantages, most aromatic amines are base-phosgenated to form isocyanates.
On the other hand, aliphatic amines, which are more strongly basic, are even more difficult to phosgenate directly as a base. Urea formation can, however, be more easily prevented if the carbamic acid salt of the amine is used in the prephosgenation step instead of the free amine.
In the so-called carbamate process, the carbamic acid salt is first prepared by saturating an amine solution with carbon dioxide as follows, EQU 2RNH.sub.2 +CO.sub.2 .fwdarw.R--N.sup.30 H.sub.3 O.sup.- OC--NH--R,
which salt is difficult to dissolve and is suspended in the solvent. In the subsequent cold phosgenation, the same carbamoyl chloride-hydrochloride mixture is formed while giving off CO.sub.2. As with the standard base phosgenation process, the cold reaction is similarly followed by hot phosgenation according to the following scheme: EQU R--NH--COO.sup.- H.sub.3 N.sup.+ --R+COCl.sub.2 .fwdarw.R--NH--COCl+R--N.sup.+ H.sub.3 C.sup.- l+CO.sub.2 EQU R--NH--CO--Cl+RN.sup.+ H.sub.3 Cl.sup.- +COCl.sub.2 .fwdarw.2R--NCO+4HCl
The above carbamate process does permit a smaller input in mixing the amine or its carbamate with phosgene. However, the disadvantage is that a third reaction stage is necessary and in addition CO.sub.2 is required which can only be recovered from the exhaust gas at considerable effort and cost.
The most effective method to prevent urea formation is to follow the so-called hydrochloride method and to phosgenate the hydrochloride a follows: EQU R--N.sup.+ H.sub.3 Cl.sup.- +COCl.sub.2 .fwdarw.RNCO+3HCl
The disadvantages of this process is that stable hydrochlorides are formed, which do not react in the cold state and even when heated their reaction rate is comparatively slow. Therefore, either long reaction times or higher temperatures necessary to reduce the duration of the reaction are required which lead to undesirable secondary reactions, such as chlorine substitution and isocyanate polymerization. Thus, it is more difficult to conduct this reaction on a continuous basis.
A typical procedure for such a phosgenation process is described in DE-OS No. 15 68 844. It is described that the hydrochloride suspension of an organic amine is phosgenated in several stages at an elevated temperature in a stirring cascade until a clear reaction solution flows off.
To illustrate the resulting difficulties, of the hydrochloride method, the phosgenation of 1,5-diamino-2-methylpentane is cited. This process is disclosed in U.S. Pat. No. 3,631,198, which indicates that the corresponding diisocyanate is isolated with a yield of only 19% under conditions in which the analogous hexamethylenediisocyanate is obtained with a 95% yield. According to the this U.S. patent, the yield can be raised to 82.7% by using diethylphthalate; however, the disadvantage of this solvent is that its boiling point is close to that of the diisocyanate which has been obtained and thus it is difficult to separate the solvent. Moreover, this solvent is not sufficiently stable under the reaction conditions described. Phthalic acid anhydride is formed which precipitates in a crystalline form and, in turn, creates a troublesome by-product.
In view of the foregoing, it is apparent that a serious need exists for an improved process for the preparation of organic isocyanates from amines.